Here, the ConfirmWindowTemplate defines the view of your pop up window. You might find handy Prism´s
Chapter 6: Advanced MVVM Scenarios, specifically the
Using Behaviors To Implement The Interaction User Experience section, where this is explained in more detail.

As for how to wire the button to a command in the view model, you could read
this thread, related on how to handle MVVM commands inside your data templates.